How do I make the camera follow two objects at .5 influence each in blender game engine?
Instead of attempting to give the camera multiple parents (which is impossible, an object can have unlimited children, but only one parent) you can put an object in between your two other objects that dynamically moves to the new average position. 

scene = bge.logic.getCurrentScene()
one = scene.objects["one"]
two = scene.objects["two"]

sense = cont.sensors["Always"]

if sense.positive:
    PosX = two.worldPosition.x + one.worldPosition.x
    PosY = two.worldPosition.y + one.worldPosition.y
    PosZ = two.worldPosition.z + one.worldPosition.z

    print(PosX, PosY, PosZ)

    own.worldPosition = (PosX/2, PosY/2, PosZ/2)`


The code looks like that. You will notice that it is simply taking the average of position on each axis and applying it to the new object. Now all you have to do is make your camera track the center object, which will dynamically place itself in the right position.

Please note that there is probably an easier way to get the average, but seeing as I am fairly terrible at python, this is the best I can come up with. THe workflow, however is the correct one.

here is the test file: